Characterizing Organic Phosphorus In Dairy Waste-Amended Soils by Enzymatic Hydrolysis.

Long-term application of animal waste to agricultural land leads to soil phosphorus (P) accumulation and an increased risk of P loss to surface waters via runoff and erosion.  Dissolved reactive P is the primary limiting factor for algal growth in freshwater ecosystems, and the transfer of P from land to streams and lakes can greatly contribute to eutrophication. However, the exact species of P present in animal waste, especially organic P, and the transformations that affect the reactivity of these various P species in soils and streams is only poorly understood. The objective of this study was to characterize enzyme-hydrolysable P along a spatial transect at a dairy operation in eastern Tennessee.  The sample transect consisted of: 1) soil from a waste application field, 2) edge-of-field soil, and 3) stream water and sediment.  Soil and dairy waste samples were taken from a 500-head dairy operation in which the waste application field lies adjacent to a 303(d) listed stream.  Surface soil from the 140-acre application field and from the adjacent riparian area of Pond Creek was sampled, as well as a control field receiving no animal waste.  Waste samples consisted of dairy lagoon waste taken from the lagoon after it had been mechanically agitated just prior to field application.  Laboratory microcosms consisting of a mixture of soil and dairy lagoon liquid were prepared.  Microcosms were incubated for 4 weeks under varying conditions.  Subsamples were taken from the microcosms weekly and analyzed for total P, Mehlich-3 extractable P, and enzyme hydrolysable P.  Enzyme hydrolyzable P of subsamples was measured by extraction with NaOH-EDTA followed by enzymatic assays.  The following enzymes were used in the assays: acid phosphatase (EC 3.1.3.2) Type 1 from Triticum aestivum (wheat germ), acid phosphotase Type IV-S from Solanum tuberosum (potato) and nuclease P1 (EC 3.1.30.1) from Penicillium citrinum (fungi).  The amount of hydrolysable P was then measured colorimetrically using a high throughput microplate-reader technique developed by Hill et al.  Model compounds containing simple monoester phosphates, condensed diester phosphates and nucleic acids were also used to validate results of enzyme hydrolysis.  Results are presented and discussed.